Fabrication process for powder composite rod

ABSTRACT

The present invention relates to a method of forming a powder rod core comprising injecting a powder mix material into a mold, moving the mold to a curing station, and injecting material into a second mold.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation-in-Part of U.S. patentapplication Ser. No. 09/782,341 filed Feb. 13, 2001 by Harold A. Sreshtaand Eric F. Drake, now pending.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention.

[0003] This invention relates to heat treatable hardfacings. Inparticular, this invention relates to a fabrication process for a powdercomposite core for subsequent use in a sheathing/compaction process formaking a high-density powder composite rod for weld-applied hardfacing.

[0004] 2. Description of the Related Art.

[0005] Hard metal overlays are employed in rock drilling bits and otherdownhole tools to form wear and deformation resistant cutting edges andfaying surfaces. These overlays comprise composite structures of hardparticles in a metal matrix. Such hard metal overlays are normallyformed by brazing or weld deposition of composite rod, producing a metalalloy matrix solidified from a melt containing hard particles thatremain at least partially solid. Early examples of hardfacing rods forwelding are shown in U.S. Pat. Nos. 1,757,601 and 3,023,130.

[0006] Hard metal overlays used on steel-toothed rolling cutter drillbits are subjected to extreme loads and prolonged scraping action.Therefore, the strongest, most wear resistant of fused hard metals areused in these cutting structures. Typically, such hard metal compositesutilize sintered pellets or grains of cemented tungsten carbide/cobaltas the primary hard phase.

[0007] In addition to steel tooth rolling cutter drill bits, other typesof down-hole tools also benefit from a strong wear resistant hardfacingmaterial. For example, fixed cutter type earth boring drill bits andstabilizers often utilize welded hardfacing to protect gauge, blade, orwatercourse surfaces. In a relatively recent development, tools made tosteer drill bits during the drilling operation provide amongst the mostdemanding applications for hardfacing materials.

[0008] The formulation of composite rod filler metal entails fabricationand process considerations in addition to constituency selection.Typically, a tubular construction has been employed wherein a metalsheath is formed to enclose a particulate mixture comprising hardparticles phases and additives including binders and de-oxidizers. Insuch a rod, the sheath metal combines with substrate melt, if any, toprovide substantially all of the matrix phase of the final composite.The constituency of the hardmetal deposit is dependent on depositionprocess parameters as well as on the raw material formulation.

[0009] The management of thermal inputs during weld deposition iscritical to deposit soundness and performance. Insufficient substrateheating and/or insufficient filler metal superheat can cause poorbonding, porosity, and irregular deposit configuration. Excess substrateheating, and/or excess filler metal superheat, and/or prolonged moltentime produces substrate dilution and hard-particle degradation.Substrate dilution reduces carbide fractions, while sintered particledegradation causes softening and matrix embrittlement.

[0010] As the carbide loading of the rod and surface area of theapplication substrate increase, weld temperature and time control becomeincreasingly critical. Composite rod with more than about 60% by weightcarbide fill is problematic to weld without substrate penetration anddilution, especially on large substrates. Deposition dynamics arestrongly influenced by the thermal transfer, melting, and flowcharacteristics of the composite rod.

[0011] Melting dynamics can be accelerated by incorporating the metalmatrix components of the composite rod as a powder rather than as asolid sheath. This approach exploits the high specific surface area ofthe particulate material to speed up melting, while eliminatingtransport and mixing dynamics. However, sheath elimination also entailsthe loss of its structural contributions to handling strength and meltprogression. U.S. Pat. Nos. 4,836,307; 4,944,774; and 5,051,112 (allincorporated by reference herein for all they disclose) disclose thesintering of a powdered composite rod as a means of replacing themechanical strength of the sheath. Such sintered pre-forms develop astrong, porous structure which acts to impede heat flow prior to meltcollapse into the weld pool. As a result, some melting speed issacrificed and melt progression becomes more difficult to control,resulting in operator-induced thickness and composition variation in thehardfacing.

[0012] In U.S. Pat. No. 4,699,848, a wire-reinforced powder rod isdisclosed, replacing external sheath with solid metal filler at thecenter of the rod, the location least easily melted by thermal flow fromexternal heat sources. This construction exacerbates weldability andcontrol limitations, compared with conventional practice.

[0013] In U.S. Pat. No. 5,740,872 (incorporated by reference herein forall it discloses) a powder composite rod is disclosed with a thin metalsheath wherein the ratio of powder metal to sheath metal exceeds 2.5.The fabrication of such a bound powder metallurgy composite rod forweld-deposited hard surfacing as described in this patent has beenconducted by extrusion and curing of rod cores, followed by sheathattachment using a wrapping mill. The rod core produced in this processhas a void volume of about 40 vol %, relying on the binder for greenhandling strength. The sheath is wrapped with a simple overlap andattached to the core by a silicate adhesive that partially infiltratesthe porous core, providing additional handling strength and preventingcore movement within the sheath. The silicate adhesive becomes a liquidslag during weld application that must be manipulated out of thedeposit, slowing application rates and demanding greater operator skill.Difficulties associated with management of this slag lend to adversedeposit effects, including pellet degradation, porosity, inclusions, andreduced thickness control. Although the thin sheath extruded rod fillermetal and application process provide net improvements in applicationproductivity, quality, and in the hardfacing performance as comparedwith conventional practice, its utility is limited by silicate adhesiveeffects and also by the relative brittleness of low-densitymethylcellulose-bound powder cores.

[0014] As mentioned above, rod cores may be fabricated by extrusion intoa trough or channel. The extruded material then is cured, such as bypartial de-binding by desiccation. It has been found that during theextrusion and desiccation of rod cores, distortions can occur whichinclude stretching, buckling, and sagging. The resulting cores exhibitbends and non-uniform section, causing difficulties in wrappingtranslating to finished rod variation that contributes to applicationvariation.

[0015] An alternative fabrication method involves injection molding ofcores into conventional split dies. This approach can substantiallyeliminate distortion but entails reduced productivity associated withprolonged core residence within the die necessary to achieve sufficienthandling strength for removal.

SUMMARY OF INVENTION

[0016] The present invention is a method for fabricating a core forincorporation into a composite rod for hardfacing.

[0017] According to the present invention there is provided afabrication process for a rod core comprising injecting rod corematerial into an elongate tubular mold, removing the mold to a curingstation, and extracting the cured rod core from the mold.

[0018] By injecting the material into a closed rigid mold, rather thanextruding it into a trough or channel, the buckling and stretchingdistortions are almost entirely eliminated. The use of a remote curingstation allows material to be injected into a second mold while thematerial within the first mold is setting up, thus speeding up themanufacturing process.

[0019] The rod core material preferably comprises a powder mix ofcarbide and metal powders, a fugitive binder, and other additives. Themold may be of, for example, circular, oval, square, hexagonal orstar-shaped cross-section.

[0020] The invention also relates to a method of fabrication of ahardfacing rod comprising encasing a rod core manufactured in accordancewith the method defined hereinbefore in a metal sheath. The method mayinclude a step of isostatically compacting the encased rod core todensify the core thereof and mechanically secure the sheath in position.

[0021] It is contemplated that the method has application to usehardfacing used in downhole tools including both fixed cutter androlling cutter drill bits, bias pads for downhole rotary steerablesystems, stabilizers, and other tools requiring strong and wearresistant hardfacings.

BRIEF DESCRIPTION OF DRAWINGS

[0022]FIG. 1 is a perspective view of a high density hardfacing rod madeby the process of the present invention.

[0023]FIG. 2 is a perspective view of a rolling cutter steel tooth drillbit with hardfacing made by the process of the present invention.

[0024]FIG. 3 is a perspective view of a fixed cutter drill bit with ahardfacing made by the process of the present invention.

[0025]FIG. 4 is a cross-section view of a tooth of a rolling cuttersteel tooth drill bit with a hardfacing applied with a high densityhardfacing rod made by the process of the present invention.

[0026]FIG. 5 is a side view of a downhole stabilizer having a hardfacingapplied with a high density hardfacing rod made by the process of thepresent invention.

[0027]FIG. 6 is a side view of a rotary steerable tool with a bias unithaving a hardfacing applied with a high density hardfacing rod made bythe process of the present invention.

[0028]FIG. 7 is a partial section view cross-section view of abottomhole assembly of a drill string with tools having hardfacingapplied with a high density hardfacing rod made by the process of thepresent invention.

[0029]FIG. 8 is a partial section view of a drill string with toolshaving hardfacing applied with a high density hardfacing rod made by theprocess of the present invention.

[0030]FIG. 9 is a diagrammatic view illustrating an apparatus for use inthe formation of rod cores.

[0031]FIG. 10 is a side view of a device which encases the cores in asteel sheath.

[0032]FIG. 11 is a perspective view of a sheet of elastomer material towrap the sheathed core prior to compaction.

[0033]FIG. 12 is a perspective view of an elastomer container with aplurality of chambers adapted to hold the undensified cured core-sheathassembly prior to compaction.

DETAILED DESCRIPTION

[0034] Referring now to FIG. 1, the high density hardfacing rod 10 ofthe present invention has a core 12 wrapped in a steel sheath 14. Thehigh density hardfacing rod 10 is particularly suitable for hardfacingsin downhole tools including earth boring drill bits 16, bias pads 18 fordownhole rotary steerable systems 20, and stabilizers 22, and for othertypes of tools requiring strong and wear resistant hardfacings.

[0035] The high density hardfacing rod 10 has a molded core 12 made of aparticulate mixture comprising cemented carbide pellets, metal carbides,metal powders, and deoxidizer, with a methylcellulose fugitive binder.

[0036] The process for making the high density hardfacing rod 10includes the steps of: preparing a powder mix comprising carbide andmetal powders, a fugitive binder, and other additives to render amoldable rheology; forming the powder mix into a powder core 12 encasedwith a metal sheath 14 to form a core-sheath assembly 110; andisostatically compacting the core-sheath assembly 110 densifying to atleast 65% of theoretical density and mechanically attaching the sheath.The core-sheath assembly 110 is dried at about 450F for between 30minutes and 2 hours, or alternatively heated in a vacuum at 930F forbetween 30 minutes and 2 hours to remove the methylcellulose binder.

[0037] The mixture is encased with a thin (0.001″ to 0.010″) steelsheath 14. This steel sheath 14 thickness range provides a weight ratioof the sheath to metallic matrix powders in the core of greater than2.5. In the preferred embodiment, the steel sheath 14 is from 0.002″ to0.006″ thick to optimize chemistry and melting characteristics.

[0038] The powder mixture consists of about 27 wt % each of {fraction(16/20)} mesh sintered WC-Co pellets and {fraction (20/30)} mesh crushedsintered WC/Co; about 9 wt %, {fraction (40/100)} mesh monocrystallineWC; about 4 wt % {fraction (40/100)} mesh cast crushed tungsten carbide;about 28 wt % 325 mesh iron powder; about 3 wt % silico-manganese powderand about 1.2 wt % methylcellulose binder.

[0039] The powder mixture is hydrated to a pH-adjusted moldable rheologyand is injection-molded to form a powder core for later encasing with asteel sheath 14. The cores 12 or core/sheath assemblies 110 aredesiccated to remove water of hydration, providing sufficient handlingstrength for subsequent processing.

[0040] As shown in FIG. 9, the hydrated powder mixture 122 is injectionmolded by using an injection unit 124 to inject the powder mixture 122into a mold 126. The mold 126 is of elongate tubular form having aninternal cross-sectional shape chosen depending upon the application inwhich the hardfacing material is to be used. For example, the internalcross-sectional shape of the mold 1 26 may be circular, oval, square,hexagonal or of star-shaped form. The mold 1 26 is dimensioned to allowfor shrinkage of the molded material, and has sufficient wall thicknessto withstand the injection pressure used which may be several thousandpsi. Alternatively, a thinner wall section may be employed ifreinforcement means are employed in the injection unit. Suchreinforcement could comprise a close-fitting rigid split die constrainedwith a closure force sufficient to carry substantially all of theinjection stress applied to the mold 126, or a hydraulically-activatedelastomeric bladder surrounding the mold 126 with balancing pressureapplied during the injection cycle.

[0041] Once the mixture 122 has been injected into the mold 126, thefilled mold 126 is moved to a curing station 128 where strengthening ofthe mixture 122 occurs preferably by desiccation. Such desiccation maybe accelerated, if desired, by heating or otherwise controlling theconditions around the mold. Once it has been determined that thematerial has cured to an extent sufficient to permit handling thereof,the core is extracted from the mold. Although this could be achieved bysplitting the mold 126 in two or more parts to permit opening thereof,it is not necessary. In the preferred embodiment, the constituency ofthe mixture 122 is adjusted such that curing shrinkage providessufficient clearance between the core and tube mold to release thematerial for gravity or ejector pin extraction.

[0042] While the mold 126 is located at the curing station 128, anothermold 130 is moved to the injection unit 124 and the mixture 122 injectedinto the mold 130. The curing station 128 is preferably designed toreceive sufficient molds to enable the manufacturing process to runcontinuously.

[0043] A device for wrapping a steel sheath 14 about a core 12 is shownin FIG. 10. A roll of steel strip 100, is pulled into a set of rolls102. Steel sheathing strip 100 may be manufactured from a variety ofmetal alloys, but is preferably manufactured from annealed low-carbonAISI 1008 steel. The rolls bend the steel strip 100 to form into a ‘U’shape. A core 104 is then placed into the ‘U’ shaped steel strip. Theassembly is further pulled through a folding die 106 and optionallythrough a reinforcing chamber (or die) 108 to form a core-sheathassembly 110. The core-sheath assembly 110 is then cut into a convenientlength, typically about 28 in., by a cut-off saw 112.

[0044] The desiccated core-sheath assembly 110 is then prepared forcompaction. Preferably, compaction of the desiccated core-sheathassembly 110 is done in a cold isostatic press. Referring now to FIGS.11 and 12, in order to achieve sufficient densification, it is necessaryto encapsulate and seal the core-sheath assemblies 110 in, for example,an elastomeric material, wrapping a sheet 114 as shown in FIG. 11 andsealing in a secondary container to encapsulate it, or preferably,encapsulating multiple core-sheath assemblies 110 in a commonelastomeric multi-cavity mold 116 shown in FIG. 12.

[0045] Encapsulated core-sheath assemblies 110 are compacted within acold isostatic press (not shown) by hydraulic compaction to a pressurebetween about 30,000 psi (30 ksi) and about 50,000 psi (50ksi)—preferably 40 ksi. The compaction cycle transforms the core-sheathassembly 110 into a high density hard facing rod 10. After compaction,the high density hard facing rod 10 reflects a wrinkled appearance dueto accommodation-buckling of the sheath as indicated by numeral 120 inFIG. 1.

[0046] The high density hard facing rod 10 is then normally dried andstored for use, or it may be treated at higher temperature to remove theresidual fugitive binder prior to application. Void content forcomposite rod with 60 wt % hardmetal is typically 40 vol % beforecompaction, decreasing to about 31 vol % after CIP-densification. Thehigh density hard facing rod 10 is about 13% more dense, with a tightlyadherent sheath, reduced oxidation susceptibility, and increasedstrength and ductility.

[0047] As shown in FIGS. 2-8, the high density hard facing rod 10 isdesigned to be applied to steel substrates, typically the surfaces ofdrill bits 16 and other downhole tools 56, 58, 62 by oxy-fuel welding(OFW). When oxygen-acetylene is utilized, flame temperature and reducingcharacteristics are established through various gas flows with aslightly oxygen reducing (excess acetylene) flame adjustment. Oxygenconsumption and application rates vary directly with the surface area,with oxygen flows varying considerably.

[0048] A hardfacing layer made from the high density hard facing rod 10exhibits well-preserved cemented tungsten carbide particles as a primaryconstituent, which retain 90% or more of their original hardness, withmonocrystalline and cast WC/WC as secondary phases. Total depositcarbide volume fractions of about 57% are typical, with porosity volumefractions less than 1%. The fraction of the sintered tungsten carbideparticles in the deposit with greater than or equal to 90% hardnessretention is greater than 84% even on large substrates, and oftenexceeds 90%. Tungsten carbide particle distribution is uniform, allowinghardfacing optimizations via site and shape control. Deposition rateincreases of 50-100% over low-density equivalents are attributed tofaster melting and the absence of silicate adhesive complications.

[0049] Tailoring of matrix properties by core powder additions or steelsheath alloy modifications is straightforward. These property andapplication advantages result from the minimization of the time that thematerials spend in the high temperature, molten state between rod 10 anddeposit. This is a direct advantage of using a powdered (but notsintered) core 12 with a very thin steel sheath 14, and OFW application,which does not gouge the substrate and provides well controlleddeposition and cooling.

[0050] The volume per cent of the cemented carbide primary particlesexhibiting 90% or more of their original hardness is at least 85%. Thesecarbide primary particles can be comprised of one or more carbides ofthe elements W, Mo, Cr, Ta, Nb, and V.

[0051] Applications for the high density hard facing rod 10 arenumerous. Referring now to FIGS. 2-4, one type of earth boring drill bit16, a tooth type rolling cutter drill bit, is shown as numeral 24 inFIG. 2. Typically, tooth bits 16 have a body 26 upon which are mountedrolling cutters 28 with cutting teeth 30. In operation weight is appliedto the bit, forcing the cutting teeth 30 of the cutters 28 into theearth 29, and, as the bit 24 is rotated, the earth 29 causes the cutters28 to rotate effecting a drilling action.

[0052] The teeth 30 are generally wedge shaped with a pair of relativelyflat flanks 32 and a crest 34. During drilling, the crest 34 is forcedinto the earth formation. By design, the rolling cutters do not allow atrue rolling action of the teeth 30 when drilling.

[0053] Therefore, each tooth 30 is scraped, or plowed in a shortdistance through the earth formation as it is penetrating the earth. Inorder to prolong the life of the drill bit a hardfacing 36 is applied tothe flanks 32 and crest 34 of the teeth 30 with the high density hardfacing rod 10 of the present invention.

[0054] Another type of drill bit 16, a fixed cutter drag-type drill bit38, is shown in FIG. 3. The fixed cutter drill bit 38 comprises a bitbody 40 machined from steel and having blades 42 formed on the leadingface 43 of the bit 38. Extending side-by-side along each of the blades42 is a plurality of cutting structures, indicated at 44.

[0055] The gauge region 46 of the drill bit 38 must resist the loadingand abrasion arising from constant scraping against the borehole wall39. Therefore, there is applied to the surface 46 a hardfacing 48 withthe high density hard facing rod 10 made by the process of the presentinvention.

[0056] Referring now to FIGS. 5 and 6, there are shown other down holetools utilizing the hardfacing applied from the high density hard facingrod of the present invention. In FIG. 5, a stabilizer 22 is shown with aplurality of blades 50. Each blade 50 must be able to withstand thesevere abrasion and loads it is subjected to during operation. In orderto extend the life of these blades, a hardfacing material 52 is oftenapplied. Hardfacing 52 applied by the high density hard facing rod madeby the process of the present invention is suitable for thisapplication.

[0057] Shown in FIG. 6, is a rotary steerable unit 20, with a bias pad18. The bias pad 18 repeatedly engages the sidewall 39 of the borehole31 in order to push the tool to one side as directed by its controlsystem. Because these bias pads 18 repeatedly apply extreme loads to theborehole wall 39, they must be coated with, or made from a very abrasionresistant and strong material such as a hardfacing 54 and applied by thehigh density hardfacing rod 10 made by a process of the presentinvention.

[0058] Referring now to FIGS. 7 and 8, are shown other applicationsutilizing downhole tools 56, 58 having a hardfacing applied with a highdensity hard facing rod made with the process of the present invention.In FIG. 8 a number of different tools 56, 58 are shown in the drillstring 60. These tools 56, 58 may include, but are not limited to,downhole motors, measuring while drilling tools, logging tools,vibration dampers, shock absorbers, and centralizes. These tools 56, 58benefit from hardfacing applied with high density hard facing rod madeby the process of the present invention. In particular, down hole bottomhole assemblies 62, as shown in FIG. 7, are often operated while gravityis pushing them against the borehole wall 39. Once again the extremeabrasion and loads applied to the size of these tools make them benefitfrom a hardfacing applied with a high density hardfacing rod made by theprocess of the present invention.

[0059] Whereas the present invention has been described in particularrelation to the drawings attached hereto, it should be understood thatother and further modifications apart from those shown or suggestedherein, may be made within the scope and spirit of the presentinvention.

What is claimed is:
 1. A method for forming a rod core for a hardfacingrod comprising injecting a powder mix material into an elongate tubularmold, moving the mold to a curing station, curing of the powder mixmaterial and removing the powder mix material from the mold.
 2. Themethod of claim 1 further comprising, after moving the mold, injectingpowder mix material into a second mold.
 3. The method of claim 1,wherein the powder mix material comprises carbide and metal powders, afugitive binder and other additives.
 4. The method of claim 3, whereinthe carbide and metal powders include refractory metal carbide, cobalt,iron and alloying material powders.
 5. The method of claim 1, whereinthe elongate tubular mold has an internal cross-sectional shape selectedfrom circular, square, oval, hexagonal and star-shaped.
 6. A method offorming a rod core comprising injecting a powder mix material into afirst elongate tubular mold, curing the powder mix material, andremoving the powder mix material from the first mold.
 7. The method ofclaim 6, further comprising injecting powder mix material into a secondelongate tubular mold while allowing curing of the powder mix materialpreviously injected into the first mold to occur.
 8. The method of claim6 further comprising, placing the first mold within a secondary supportstructure which closely fits about the mold, prior to injecting thepowder mix material into the mold.
 9. The method of claim 8 wherein thesecondary support structure comprises a rigid split die or a fluid diethat introduces external pressure on the mold during the injectioncycle.
 10. A method of forming a rod core comprising supplying a powdermix material into an elongate mold, curing the powder mix material, andsupplying powder mix material to a second elongate mold while curingproceeds in the first mold.